STUDIES carried on by the writer since 1926 with plants illuminated both day and night
seem of sufficient interest to report upon briefly at this time. Others who
have tried somewhat similar although not exactly the same experiments1 seem not
to have secured just the results which have been so apparent in my work.
Preliminary accounts of my studies were made at meetings of the Southwestern
Division of the American Association for the Advancement of Science in Santa
Fe, 1927, and in Flagstaff, 1928. The experiments are being continued, and a
full account will be published at a later time.

Plants,
chiefly annuals, have been grown in the greenhouse under natural light in the
daytime and, in addition, during both day and night they have had the light of
two 100-watt Mazda lamps suspended above the bench at a distance of four feet-the
lamps provided with an overhead reflector. Controls, shielded from the
artificial light, are growing in the same room of the greenhouse on the same
bench at a distance of about ten feet. A total of nearly one hundred species
have been worked with, some of them 'during two or more seasons if first
results seemed doubtful. The list includes common garden vegetables, grains,
weeds, native herbs and garden ornamentals.

In
general, the experimental plants are taller than the controls at all times
during the entire growth period, this increased height being due to elongation
of internodes. Frequently the experimental plants are slender-stemmed and have
a decumbent habit. Flowering is usually hastened under continuous light but in
a few species is completely inhibited. Plants of some species reach full adult
stature, come to blossom, and produce fruit and seed while the check plants are
still in the rosette stage close to the ground.

The
root system in plants of the experimental series is invariably less extensive
than that of the controls; roots are smaller, shorter, and have fewer branches.
Thickened taproots do not develop.

Leaves
of plants under continuous illumination often show no modification but in a
considerable proportion of the species studied they have smaller, thinner
blades, and often longer petioles. Leaves of monocotyledons tend to be very
much lengthened as do the sinuately deeply-cleft leaves of certain
Hydrophyllaceae. Reduction in leaf size is especially noticeable in certain
members of the chickweed family, and this with the thin stems and greatly
lengthened internodes and frequent paleness of color gives a suggestion of
etiolation. But only a few species are sufficiently pale or show the leaves so
much reduced as to make the similarity to etiolated plants very pronounced. The
greatly lengthened internodes are, however, a practically constant feature.

Internally,
the stems of plants grown under continuous light show a thinner cortex, less
vascular tissue (especially phloem), and a relatively larger pith than the
controls. Leaf-blades in cross-section look as if derived from plants grown in
the shade, usually having a single layer of palisade and with more and larger
intercellular spaces than the plants grown under ordinary greenhouse
conditions. Leaf cells are smaller, hence the leaves are thinner. Roots of the
plants of the experimental series show slight development of phloem but
otherwise are of usual structure, except that as previously noted they are
small and short and with few branches.